Integrity of Mild Steel Structures at -80¦C

[JMOwen]

Not strictly a cryogenic application but..

I am currently evaluating the design of some large but lightly stressed welded storage frames. We have many years of experience of similar designs fabricated in mild steel (S275/43A) operating in a -20°C environment.

My concern is that that the new generation of frames will be in service at -80°C and must have a 25 year design life.

Brittle fracture is obviously the main concern. There is no stress cycling to speak of of fatigue is unlikely to be an issue. Specifiying 304 stainless steel would solve the problem but adds significant cost. Aluminium is prohibitively expensive because of the welding costs.

Does any one know of other less expensive low temperature steels that are readiliy available?

Alternately is there any data on the fracture toughness / stress limits for mild steels at -80°C?

 

[Carburize]

There may be data but I would not be too difficult to run some tests on your own material including any welds you might be going to use. Fracture toughness testing will cost you around $1000 per set of three. Once you have the fracture toughness numbers you can plug in the stress levels and calculate some tolerable defect sizes, these tell you the size of flaw which effectively has to be inspected out of the structure for it to operate safely under your design stresses at the opeating (test) temperature. If the numbers come out to be of the order of an inch generally speaking you do not have a brittle fracture issue, if they come out less than about 0.125-inch you will need to consider a different material. These are very general numbers and are obviously highly dependent on the stresses in your structure. My company makes a machine using regular structural steels with no special controls on low temperature properties which operates very successfully at -40C with no issues concerning brittle fracture because the stresses are low and the tolerable defect sizes are large.